Attention Deficit Hyperactivity Disorder (ADHD) is a serious, common, and chronic behavioral syndrome that is a risk factor for negative outcomes in both academic and social domains. Performance variability, as measured by intra-individual variability in reaction times (RTs), has largely been ignored as "noise" within data of interest when looking at cognitive deficits associated ADHD. However, greater than expected variation in performance is now being recognized as one of the most prominent features of the disorder and has been linked to possible etiological mechanisms underlying ADHD. However, intra-individual variability remains poorly defined, and although some evidence suggests that RT variability in ADHD is created by predictable low-frequency fluctuations in performance that that can be identified in frequency domain analyses (Castellanos et al., 2005;Johnson et al., 2007), it is unclear whether heightened variability is a stable feature of cognitive processing or whether contextual factors, such as reward, may be able to modulate it. Using a go/no-go task, which taps both response inhibition and sustained attention, the current study will assess intra-individual variability in RTs in children with and without ADHD, at baseline and in reward conditions, to test hypotheses that: 1) children with ADHD will show increased RT variability that is driven by long reaction times occurring several times each minute and 2) variability in specific low-frequency bands will be reduced for all children, but especially so for children with ADHD, by the introduction of salient rewards. Results of the proposed study have implications for the on-going efforts to assess cognitive deficits in ADHD, as well as for understanding what interventions may best mitigate cognitive deficits in this group. In addition, given that intra-individual variability has been proposed as a possible endophenotype of the disorder, suggesting that it is linked to the biological dysfunction underlying ADHD, results also will have implications for understanding the etiology of the disorder.